The present invention relates to a thermal pressure fixing device for a copying machine, comprising a pressure roll and a fixing roll for heating and pressing an image receiving material, which carries a toner image. During the passage of the image receiving material between the two rolls, the latter are pressed against each other, and this pressure contact is neutralized when the copier is shut off or a malfunction occurs within the copier.
Such a thermal pressure fixing device, comprising a pair of rolls for heating and pressing a copying material carrying a toner image, is described in German Offenlegungsschrift No. 26 52 731. In this device, a toothed control wheel is provided which mates with a toothed planetary wheel rotating around the toothed control wheel. The toothed planetary wheel is driven by a special mechanism, and there is also a mechanism provided which reacts to the rotary movement of the toothed planetary wheel, in order to shift the pair of rolls between a first position where the rolls are pressed against each other, and a second position where the pressure is released. This known fixing device has three rolls, namely a fixing roll, a heating roll and a pressure contact roll. The two latter rolls are equipped with internal heating means. This device solves the problem of releasing the pressure during a standstill or in case of a malfunction of the copier; however, the rolls are not separated from each other. Rather, the heated rolls and the fixing roll remain in a slight, pressureless contact, even when no fixing process goes on, whereby the fixing roll is uniformly heated by the two other rolls and, is kept at a predetermined temperature.
A complete separation of the rolls is not provided, nor is there taken into consideration a change in the external diameters of the silicone rubber coated or tetrafluoroethylene resin (Teflon) coated rolls. Despite constant center distances of the rolls, the roll pressure may--due to swelling of the coating materials--increase to such an extent that even during the standstill of the copier, i.e., when the rolls just slightly contact each other, pressures occur which are so high as to lead to a considerably shortened operational life of the rolls.
German Offlegungsschrift No. 27 58 245 describes a thermal pressure fixing device, wherein a slight pressure contact between a pressure roll and a fixing roll is maintained by associated means while the copier is switched off, and also at the moment, or up to the moment where it is switched on. The pressure roll moves vertically up and down, and the ends of its shaft are supported by bearing elements in such a way that the weight of the roll is carried by pressure levers. Below the pressure levers are provided cam-type, curved bearing surfaces which are in contact with the undersides of the levers. When no fixing is being carried out, the levers contact the flat sections of the curved surfaces. During the fixing process, the levers contact the higher, i.e., the steeper sections, of the curved surfaces, and thus press the pressure roll against the cylindrical surface of the heating roll at a high compressive force.